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A Cosmic Hall of Mirrors

Article in Physics World · October 2005

DOI: 10.1088/2058-7058/18/9/28 · Source: arXiv

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 A cosmic hall of mirrors

Jean-Pierre Luminet
Laboratoire Univers et Théories (LUTH) – CNRS UMR
Observatoire de Paris, 92195 Meudon (France)
Jean-pierre.luminet@obspm.fr

Abstract

Conventional thinking says the universe is infinite. But it could be finite and relatively small,
merely giving the illusion of a greater one, like a hall of mirrors. Recent astronomical
measurements add support to a finite space with a dodecahedral topology.

Introduction

For centuries the size and shape of the universe has intrigued the human race. The Greek
philosophers Plato and Aristotle claimed that the universe was finite with a clear boundary.
Democritus and Epicurus, on the other hand, thought that we lived in an infinite universe
filled with atoms and vacuum. Today, 2500 years later, cosmologists and particle physicists
can finally address these fundamental issues with some certainty.
Surprisingly, the latest astronomical data suggest that the correct answer could be a
compromise between these two ancient viewpoints: the universe is finite and expanding but it
does not have an edge or boundary. In particular, accurate maps of the cosmic microwave
background – the radiation left over from the Big Bang -- suggest that we live in a finite
universe that is shaped like a football or dodecahedron, and which resembles a video game in
certain respects.
In such a scenario, an object that travels away from the Earth in a straight line will
eventually return from the other side and will have been rotated by 36 degrees. Space might
therefore act like a cosmic hall of mirrors by creating multiple images of faraway light
sources, which raises new questions about the physics of the early universe. However, this is
just one possibility and other proposals made by researchers in the expanding field of cosmic
topology include tetrahedral and octahedral spaces, flat doughnuts and an infinite "horn-
shaped" universe.

The curvature of space

The first testable predictions about the size and shape of the universe were made by Einstein
in 1916 as part of his general theory of relativity. In general relativity massive bodies such as
stars change the shape of space--time around them, much as a bowling ball would change the
shape of a trampoline. Indeed, it is this local deformation of space--time that is responsible for
gravity in Einstein's theory.
The average curvature of space therefore depends on the overall density of matter and
energy in the universe. This density is usually expressed in terms of the parameter !, which is
defined as the ratio of the actual density to the critical density required for space to be flat or
Euclidean. Space can therefore have three possible curvatures: zero curvature (! = 1), which
means that two parallel lines remain a constant distance apart as they do in the familiar
Euclidean space; negative curvature (! < 1), with parallel lines diverging as they do on the
hyperbolic surface of a saddle; or positive curvature (! > 1), which means that parallel lines
eventually cross one another as they do on the surface of a sphere.
In the standard model of cosmology space has been flat and infinite ever since the universe
underwent a short period of extremely rapid expansion called inflation shortly after the Big
Bang. Moreover, we now know that the expansion of the universe is actually accelerating due
to a mysterious repulsive force caused by "dark" energy (see Physics World May 2004 pp37--
42).
In 2003 the Wilkinson Microwave Anisotropy Probe (WMAP) produced a high resolution
map of the cosmic microwave background that provided clues about the expansion rate of the
universe and its curvature. Combined with other astronomical observations the WMAP data
suggest that ! = 1.02 ± 0.02, which favours a spherical universe with positive curvature. The
simplest such space is a hypersphere, which can be thought of as the 3D surface of a 4D ball,
just as an ordinary sphere is the 2D surface of a 3D ball. Hyperspherical space is therefore
finite but it does not have a problematic boundary (figure 1). However, as we will see, many
other spherical spaces can fit the data better than a hypersphere.

The topology of space

Curvature is clearly central to the large-scale shape of space, but it is not the only factor. The
global topological properties of space are also important because they determine whether the
universe is finite or infinite. All spherical spaces are finite, for instance, but not all finite
spaces are spherical. Indeed, flat and hyperbolic spaces can have finite or infinite volumes
depending on their topologies.
To illustrate this in two dimensions, think of a square and identify opposite sides as being
the same, as happens in video games where a spaceship disappearing to the right of the screen
reappears on the left. In three dimensions a spaceship or anything else (such as a particle or a
photon) that leaves the "fundamental" cube through one face re-enters it from the opposite
face. In this case one can imagine a cubical block of space whose opposite faces have been
"glued" together to produce what is effectively a 3D torus.
At first glance all the familiar rules of Euclidean geometry hold in both of these examples,
and the spaces looks infinite to those who live in them. However, unless the spaceship keeps
encountering the same objects on its travels it has no way of telling if it is moving through an
infinite space or through the same finite space again and again.
General relativity does not distinguish between these possibilities because each of the three
plausible cosmic geometries -- flat, hyperbolic and spherical -- is consistent with many
different topologies. For example, a 3D torus and ordinary flat Euclidean space are described
by the same equations in general relativity, even though the former is finite and the latter
infinite. Determining the topology of the cosmos therefore requires some physical
understanding beyond relativity, in particular concerning the way different regions of space--
time are connected.
Cosmologists usually assume that the universe is simply connected like a plane, which
means there is only one direct path for light to travel from a source to an observer. A simply-
connected Euclidean or hyperbolic universe would indeed be infinite, but if the universe is
multiply-connected like a torus there would be many different possible paths. This means that
an observer would see multiple images of each galaxy and could easily misinterpret them as
distinct galaxies in an endless space, much as a visitor to a mirrored room has the illusion of
seeing a crowd. Could we, in fact, be living in such a cosmic hall of mirrors?
Topologists have proved that in addition to the ordinary, simply-connected Euclidean,
spherical and hyperbolic spaces there are 17 other Euclidean spaces and an infinite number of
spherical and hyperbolic spaces -- all of which are multiply connected. These spaces differ in
the shape of their fundamental blocks, which could take the form of a parallepiped or a
hexagonal prism for a Euclidian space or more complicated polyhedrons for spherical and
hyperbolic spaces. The way the faces of these blocks are glued together also differs between
each space. The surprise from the WMAP data is that the topology of space seems indeed to
be multiply connected, and described by a special class of shapes that are called "well
proportioned".

At a glance : Cosmic topology

• There are three possibilities for the curvature of the universe: space can be flat,
spherical or hyperbolic.
• The geometry of the universe depends on its curvature and also on its topology,
which governs the way space is connected and so determines if it is finite or
infinite
• Measurements of the cosmic microwave background constrain the curvature of
the universe and provide hints about its topology
• Recent data suggest that the universe might be multiply connected, like the left-
and right-hand sides of the screen in a computer game
• Since the early 1990s the number of cosmo-topologists around the world has
grown to more than 50

Cosmic harmonics

The best way to determine the shape of our universe is to go back to its beginning, just after
the Big Bang. The infant universe is thought to have been crossed by real acoustic waves that
would have caused tiny density fluctuations in the primordial plasma. After about 380::000
years, however, the universe had expanded and cooled enough to allow hydrogen and helium
to form. This meant that photons could suddenly travel unhindered through space, carrying
with them vital information about the primordial density fluctuations (which are now thought
to have been the seeds for galaxies and clusters of galaxies to form). Today, 13.7 billion years
after the Big Bang, this radiation has cooled to a temperature of about 2.7 K, which is in the
microwave region. And the fluctuations are imprinted as hot and cold spots in this cosmic
microwave background.
A good way to understand the connection between acoustics and topology is to sprinkle
fine sand uniformly over a drumhead and then make it vibrate. The grains of sand will collect
in characteristic spots and patterns that reveal information about the local geometry of the
drum and about the elasticity of its membrane. But the distribution of spots also depends on
the global shape -- i.e. the topology -- of the drum. For example, the waves will be reflected
differently according to whether the drumhead is infinite or finite, and whether it is shaped
like a circle, an ellipse or some other shape.
Just as the vibration of a drumhead may be expressed as a combination of its harmonics,
fluctuations in the temperature of the cosmic background radiation may be expressed as
combinations of the vibrational modes of space itself. When the level of fluctuations is plotted
as a function of angle we therefore find a series of peaks that provide a signature of the
geometry of space 13.7 billion years ago (figure 2). For example, the position and amplitude
of the first peak -- i.e. the peak at the largest angle -- in this "angular power spectrum" gives
the radius of curvature of space.
Different cosmological models predict different power spectra, and high-resolution
measurements of the cosmic microwave background from instruments such as WMAP now
allow us to compare different theories against real data. However, when WMAP released its
first data in 2003, tenants of the standard cosmological model were faced with several
surprises.
The position of peaks in the angular spectrum is usually described by their wavenumber or
mode l = 180°/", where " is the angular distance in the sky. In fact, the lowest l = 1 or
"dipole" mode is swamped by the far stronger dipole induced by the motion of the solar
system relative to the cosmic background, which means that it cannot be measured. But when
researchers determined the first observable mode -- the l = 2 "quadrupole" -- they found that
it was seven times weaker than expected for a flat, infinite universe. Furthermore, the
"octopole mode" with l = 3 was also found to be weaker than the expected value, by a factor
of about two thirds.
For higher modes up to l = 900, which correspond to angular scales of just 0.2°, the
WMAP data were fairly consistent with the standard model. But a more careful analysis of the
power spectrum also revealed that the distribution of temperature fluctuations is not fully
isotropic and that the fluctuations are distributed differently on different angular sales.
All these anomalies contradict the standard picture of the universe, which has led some
more conservative cosmologists to claim that they are due to bad data analysis. Furthermore,
the second round of WMAP data -- originally expected in February 2004 -- has been delayed
for more than a year, which may hint at additional trouble to come! Meanwhile, other
cosmologists have taken the problem seriously and proposed new laws to explain the early
universe, some of which have exotic names such as “vanilla” and “racetrack” inflation.
Cosmo-topologists, on the other hand, have tried to find a more natural geometrical
explanation for the observed power spectrum. Put simply, the unusually low amplitudes of the
quadrupole and octopole modes means that long wavelengths (i.e. temperature fluctuations
over large angular scales) are missing, which could simply be because space is not big enough
to sustain them. This can be likened to oscillation of a string fixed at both ends, where the
maximum wavelength of an oscillation is twice the string length. The geometrical explanation
of the power spectrum thus implies that we line in a finite, multiply-connected space that is
smaller than the observable universe.

Dodecahedral space

Surprisingly, not all small-volume universes suppress the large-scale fluctuations. In 2003 the
present author, Jeff Weeks and co-workers proved that the long-wavelength modes tend to be
relatively lowered only in a special family of finite, multiconnected spaces that are called
“well-proportioned spaces” because they have a similar extent in all three dimensions. More
specifically, we discovered that the best candidate to fit the observed power spectrum is a
well-proportioned space called the Poincaré dodecahedral space.
This space may be represented by a polyhedron with 12 pentagonal faces, with opposite
faces being “glued” together after a twist of 36° (figure 3). This is the only consistent way to
obtain a spherical (i.e. positively curved) space from a dodecahedron: if the twist was 108°,
for example, we would end up with a radically different hyperbolic space. The Poincaré
dodecahedral space is essentially a multiply connected variant of a simply connected
hypersphere, although its volume is 120 times smaller.
A rocket leaving the dodecahedron through a given face immediately re-enters through the
opposite face, and light propagates such that any observer whose line-of-sight intercepts one
face has the illusion of seeing a slightly rotated copy of their own dodecahedron. This means
that some photons from the cosmic microwave background, for example, would appear twice
in the sky.
The power spectrum associated with the Poincaré dodecahedral space is different from that
of a flat space because the fluctuations in the cosmic microwave background will change as a
function of their wavelengths. In other words, due to a cut-off in space corresponding to the
size of the dodecahedron, one expects fewer fluctuations at large angular scales than in an
infinite flat space, but at small angular scales one must recover the same pattern as in the flat
infinite space. In order to calculate the power spectrum we varied the mass–energy density of
the dodecahedral universe and computed the quadrupole and the octopole modes relative to
the WMAP data. To our delight, we found a small interval of values over which both these
modes matched the observations perfectly. Moreover, the best fit occurred in the range 1.01<
# <1.02, which sits comfortably with the observed value.
The Poincaré dodecahedral space therefore accounts for the lack of large-scale fluctuations
in the microwave background and also for the slight positive curvature of space inferred from
WMAP and other observations. Moreover, given the observed values of the mass–energy
densities and of the expansion rate of the universe, the size of the dodecahedral universe can
be calculated. We found that the smallest dimension of the Poincaré dodecahedron space is 43
billion lightyears, compared with 53 billion light-years for the “horizon radius” of the
observable universe. Moreover, the volume of this universe is about 20% smaller than the
volume of the observable universe. (There is a common misconception that the horizon radius
of a flat universe is 13.7 billion light-years, since that is the age of the universe multiplied by
the speed of light. However, the horizon radius is actually much larger because photons from
the horizon that are reaching us now have had to cross a much larger distance due to the
expansion of the universe.)
If physical space is indeed smaller than the observable universe, some points on the map of
the cosmic microwave background will have several copies. As first shown by Neil Cornish
of Montana State University and co-workers in 1998, these ghost images would appear as
pairs of so-called matched circles in the cosmic microwave background where the
temperature fluctuations should be the same (figure 4). This “lensing” effect, which can be
precisely calculated, is thus purely attributable to the topology of the universe.
Due to its 12-sided regular shape, the Poincaré dodecahedral model actually predicts six
pairs of diametrically opposite matched circles with an angular radius of 10–50°, depending
on the precise values of cosmological parameters such as the mass–energy density.

Circles in the sky

When news of our dodecahedral model appeared in Nature in October 2003, it was not long
before the press started running headlines based on what was being hailed the new “football-
shaped” model of the universe. However, since cosmo-topology is a very competitive
field, the initial response from other groups was not always favourable.
For instance, the New York Times ran the headline “Cosmic soccer ball? Theory already
takes sharp kicks”, based on an apparently negative search for matched circles in the WMAP
data performed by Cornish and coworkers. Using massive computer simulations, they claimed
to have found no evidence of matching on angular sizes greater than 25° and thus rejected the
Poincaré hypothesis the same day it appeared.
In fact, their rejection was rather premature because they had only looked for non-rotated
matched circles that were diametrically opposite one another – a case that did not test the
dodecahedron model at all. After the initial excitement, Cornish and co-workers went back
and reassessed the data. Taking account of the additional 36° twist took a few additional
months of computer time, but the matched circles remained elusive. This led them to conclude
that there was no reasonable topology for the universe that had a characteristic length smaller
than the observable horizon.
However, it turned out that the researchers had taken a short cut to save computer time. While
they correctly took into account the possible rotations between matched circles that are
implied by most multi-connected topologies, they only searched for matched circles that were
back-to-back or very nearly back-to-back. This led them to exclude all likely multiply
connected spaces. In the mean time, however, we had proved that in most multiply connected,
well-proportioned topologies space is not homogeneous. This means that the position of
matched circles in the sky depends on the location of the observer, and they are not, therefore,
back-to-back. Only in the simplest of topologies, such as the hypertorus in flat space and the
Poincaré dodecahedron in a spherical space, is space homogeneous and the circles back-to-
back.
This violates one of the most basic principles of cosmology, that there is no privileged
position in the universe. But this principle could be illusory, like the ant in the desert that is
convinced the whole world is filled with sand and dunes. For instance, in a flat-torus universe,
any gluing together of the opposite faces combined with a screw motion produces pair of
circles that are far from being back-to-back. Unfortunately, the increase in the number of
degrees of freedom that results from such a scenario means that a full-circle search in the
WMAP data is beyond current computing capabilities.

Cosmic horn

In June 2004, however, Boud Roukema and colleagues at the Torun Centre for Astronomy in
Poland independently searched for circles in the WMAP data. By only looking for back-to-
back circles within a limited range of angular sizes and neglecting all other possible matches,
the computer time was reduced drastically. Remarkably, the Polish team found six pairs of
matched circles distributed in a dodecahedral pattern and twisted by 36°, each with an angular
size of about 11°. This implied that # = 1.010 ± 0.001, which is perfectly consistent with our
dodecahedral model, although the result was much less publicized than the earlier negative
results.
In fact, the statistical significance of the match still needs to be improved, which means
that the validity of the Poincaré dodecahedron model is still open to debate. In the last few
months, however, there has been much theoretical progress on well-proportioned spaces in
general. Early this year, for example, Frank Steiner and co-workers at the University of Ulm
in Germany proposed a multiply connected hyperbolic topology called the Picard hyperbolic
space. Like the Poincaré dodecahedron, this horn-shaped space belongs to the family of well-
proportioned spaces and it also correctly fits the low vibrational modes of the WMAP data.
However, since the topology requires the density parameter to have a value of # = 0.95, and
thus a negatively curved space, it does not fit the experimental constraints we already have on
the curvature of space.
After studying the horn-shaped topology further, Steiner and co-workers realized that well-
proportioned spherical spaces were, in fact, more promising. They went on to prove that the
fit between the power spectrum predicted by the Poincaré dodecahedron model and that
observed by WMAP was even better than we had previously thought. But the German team
also extended its calculations to well-proportioned tetrahedral and octahedral spherical spaces
in which # > 1 (see figure 3).
These spaces are somewhat easier to understand than a dodecahedral space, but they
require higher values of the density: # > 1.015 for octahedral spaces and #>1.025 for
tetrahedral spaces, compared with # > 1.009 for dodecahedral spaces. However, these values
are still compatible with the WMAP data. Furthermore, Steiner and co-workers found that the
signal for pairs of matched circles could have be missed by current analyses of the cosmic
microwave background due to various measurement effects that damage or even destroy the
temperature matching.
 Another active area of cosmic topology is “cosmic crystallography”, which was initially
devised by the present author and co-workers in 1996 and is now being pursued by, among
others, Germán Gomero of the Universidade Estadual Paulista in Brazil and Marcelo
Reboucas of the Brazilian Center for Research in Physics. In cosmic crystallography
researchers look for repeating patterns in the 3D distribution of high-redshift sources, such as
galaxy clusters and quasars, much like the repeating patterns of atoms observed in crystals.
By building so-called pair-separation histograms, cosmologists are in most cases able to
detect a multiconnected topology of space in the form of spikes that clearly stand out above
the distribution expected for the simply connected case.

A Pandora’s box for physics

Finite well-proportioned spaces, especially the Poincaré dodecahedron, open something of a

Pandora’s box for the physics of the early universe. The standard model of cosmology relies
in the main on the hypothesis that the early universe underwent a phase of exponential
expansion called inflation, which produced density fluctuations on all scales. In the simplest
inflationary models, space is supposed to have become immensely larger than the observable
universe. Therefore, a positive curvature (i.e. # > 1), even if weak, implies a finite space and
sets strong constraints on inflationary models.
It is possible to build “low scale” inflationary universes in which the inflation phase ends
more quickly than it does in general inflationary modes, leading to a detectable space
curvature. In other words, even if space is not flat, a multiconnected topology does not
contradict the general idea of inflation. However, no convincing physical scenario for this has
yet been proposed.
Perhaps the most fundamental challenge is to link the present-day topology of space to a
quantum origin, since general relativity does not allow for topological changes during the
course of cosmic evolution. A quantum theory of gravity could allow us to address this
problem, but there is currently no indication about how such a unified theory might actually
describe the emergence of multiply connected spaces.
Data from the European Planck Surveyor, which is scheduled for launch in 2007, will be
able to determine # with a precision of 1%. A value lower than 1.01 will rule out the Poincaré
dodecahedron model, since the size of the corresponding dodecahedron would become greater
than the observable universe and would not leave any observable imprint on the microwave
background. A value greater than 1.01, on the other hand, would strengthen the models’
cosmological pertinence.
Whether or not some multiply connected model of space such as the Poincaré
dodecahedron is refuted by future astronomical data, cosmic topology will continue to remain
at the heart of our understanding about the ultimate structure of our universe.

References

W Aurich et al. 2005 CMB anisotropy of the Poincaré dodecahedron, arXiv.org/abs/astro-

ph/0412569.
N Cornish et al. 2004 Constraining the topology of the universe, Phys. Rev. Lett. 92 201302.
M Lachièze-Rey & JP Luminet 1995 Cosmic Topology Phys. Rep. 254 135.
J Levin 2002 Topology and the cosmic microwave background Phys. Rep. 365 251
J P Luminet et al. 2002 Is space finite? The Once and Future Cosmos, Scientific American
(special edition) pp58–65.
J P Luminet et al. 2003 Dodecahedral space topology as an explanation for weak wide-angle
temperature correlations in the cosmic microwave background Nature 425 593.
J P Luminet 2005 L’Univers chiffonné (Paris, Fayard)
A Riazuelo et al. 2004 Cosmic microwave background in multiconnected flat spaces Phys.
Rev. D 69 103518.
B Roukema et al. 2004 A hint of Poincaré dodecahedral topology in the WMAP first year sky
map Astron. Astrophy. 423 821
J Weeks 2001 The Shape of Space (New York, Dekker)
WMAP results: map.gsfc.nasa.gov
 Lost in space
A multiconnected dodecahedral universe gives the impression of being 120 times
bigger than it actually is, similar to a hall of mirrors [Credit: Jeff Weeks].
 Figure 1: How flat is the universe?
The curvature of space and our horizon radius is determined by the average
density of the universe and its expansion rate. Cosmologists often say that space is
nearly flat because the observed value of the density is close to the critical value
for a flat universe. However, if the density was 2% more than the critical density,
the horizon radius (red line) would be about 46 billion light-years and the radius
of curvature of the corresponding hypersphere would be only 2.6 times greater.
We would therefore see a modest but non-trivial portion of the hypersphere. If the
density is exactly equal to the critical value, space is Euclidean, the radius of
curvature is infinite and we can only see an infinitesimal portion of the universe.
 Figure 2: Angular power spectrum
The evidence that space might have an unusual shape can be seen when we plot
the level of temperature fluctuations in the cosmic microwave background as a
function of angle, " (the peaks in the spectrum corresponds to modes l = 180°/").
In particular, the largest angular scales (or lowest modes) are very sensitive to the
curvature of space (inset). In this region data from WMAP (black dots) do not
agree with theoretical predictions for an infinite Euclidean space (red curve),
whereas predictions based on a Poincaré dodecahedral space (blue curve) do
agree. This suggests that we live in a multiply-connected spherical space.
 Figure 3: The many shapes of the universe
The Poincaré dodecahedral space (left) can be described as the interior of a
"sphere" made from 12 slightly curved pentagons. However, this shape has a big
difference compared with a football because when one goes out from a pentagonal
face, one comes back immediately inside the ball from the opposite face after a
36° rotation. Such a multiply connected space can therefore generate multiple
images of the same object, such as a planet or a photon. Other such spaces that fit
the WMAP data are the tetrahedron (middle) and octahedron (right). [Credit: Jeff
Weeks]
 Figure 4: Simulated circle matching
The topology of the universe describes how different regions are connected and
should therefore leave its imprint on the cosmic microwave background. For
example, if our physical space is smaller than the observable universe (as recent
data suggest it is) then the horizon sphere wraps around the universe and intersects
itself. As a result, duplicated images of the cosmic microwave background (in
which the colours represent temperature fluctuations) will intersect along a circle
and we would observe this circle on different sides of the sky. [Credit: A
Riazuelo]

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